1. Field of the Invention
The present invention relates to an image forming apparatus and an image reading apparatus and, more particularly, to an image forming apparatus and an image reading apparatus for the image forming apparatus for performing image formation using a recording head having an array of a plurality of recording elements.
More particularly, the present invention relates to an apparatus having a mechanism for automatically adjusting print characteristics of a recording head in an ink-jet recording apparatus and is especially effective in an apparatus for forming a multi-gradation color image by overlapping ink droplets.
2. Related Background Art
Along with development of information processing equipment (e.g., a copying machine, a wordprocessor, and a computer) and communication equipment, apparatuses for performing digital image recording using a recording head by using an ink-jet scheme, a thermal transfer scheme, or the like have been very popular as image forming (recording) apparatuses for equipment such as information processing equipment and communication equipment. In such a recording apparatus, a recording head having an array of a plurality of recording elements (to be referred to as a multi-head hereinafter) is generally used to increase a recording speed.
For example, a multi-nozzle head having a plurality of ink orifices and a plurality of liquid paths is generated used in an ink-jet recording head. A plurality of heaters are generally stacked in a thermal transfer or thermal head.
It is very difficult to uniformly manufacture recording elements of a multi-head due to variations in characteristics during the fabrication process and variations in properties of head constituting materials. Variations occur in the characteristics of the respective recording elements. For example, in the above multi-nozzle head, variations occur in shapes of the orifices and liquid paths. In the above thermal head, shapes of heaters and resistances inevitably vary. Nonuniformity of characteristics between the recording elements appears as nonuniformity of sizes of dots recorded by the respective recording elements and as uneven image densities of the resultant image.
To cope with the above problems, various methods are proposed in which an uneven image density is visually found or an adjusted image is visually checked, a signal applied to each recording element is manually corrected, thereby obtaining a uniform image.
For example, in a multi-head 330 having recording elements 331 arranged as shown in FIG. 1A, assume that input signals to the respective recording elements are uniform, as shown in FIG. 1B, and that an uneven image density is visually found, as shown in FIG. 1C. In this case, an input signal is corrected, as shown in FIG. 1D. More specifically, a large input signal is supplied to a recording element corresponding to a low image density, and a small input signal is supplied to a recording element corresponding to a high image density, thereby generally performing manual adjustment.
In a recording scheme capable of modulating a dot diameter or dot density, the diameter of a dot to be recorded by each recording element is modulated in accordance with an input to achieve gradation recording. For example, if modulation of a drive voltage applied to each injection energy generating element (e.g., a piezoelectric element or an electricity-heat conversion element) in an ink-jet recording head according to a piezoelectric scheme or a scheme utilizing heat energy, or a drive voltage applied to each heater in a thermal head or a pulse width of the drive voltage in accordance with an input signal is utilized, a dot diameter or a dot density of each recording element can be made uniform, and a density distribution can be made uniform, as shown in FIG. 1E. Alternatively, assume that it is difficult to modulate a drive voltage or pulse width or it is difficult to perform density adjustment in a wide range even if the drive voltage or pulse width is modulated. In this case, if one pixel is constituted by, e.g., a plurality of dots, the number of dots to be recorded in accordance with an input signal is modulated. A larger number of dots are assigned to a portion having a low density, while a smaller number of dots are assigned to a portion having a high density. If one pixel is constituted by one dot, the number of orifice-use times (the number of injection cycles) per pixel is modulated in the ink-jet recording apparatus, thereby changing the dot diameter. Therefore, the density distribution can be made uniform, as shown in FIG. 1E.
Japanese Patent Laid-Open Application No. 57-41965 filed by the present applicant discloses that a color image is automatically read by an optical sensor, and a correction signal is supplied to each ink-jet recording head to form a desired color image. In this prior-art invention, basic automatic adjustment is disclosed, and an important technique is thus disclosed. Various problems may be posed when this prior-art invention is embodied in a variety of practical applications as various apparatuses. However, technical problems addressed in the present invention are not solved in this prior-art invention.
Techniques except for a density detection scheme are disclosed in Japanese Laid-Open Patent Application No. 60-206660, U.S. Pat. No. 4,328,504, and Japanese Patent Laid-Open Application Nos. 50-147241 and 54-27728. A landing position of a liquid droplet is automatically read, and the read position data is corrected to perform landing to an accurate position. Although these schemes are common as automatic adjustment techniques, the technical problems addressed in the present invention are not solved in these prior-art inventions.
According to the above method, even if an uneven image density can be corrected once, a correction quantity of an input signal must be changed when the uneven image density is changed. In an ink-jet head, a precipitate of an ink may be attached to a portion near an ink orifice, or a foreign substance may be attached thereto, and the density distribution may often be changed. In thermal transfer, degradation of each heater and its change of properties occur to result in a change in density distribution. In this case, an initial input correction quantity does not allow sufficient uneven image density correction. For this reason, an uneven image density becomes conspicuous in long-term use, resulting in inconvenience.
In order to cope with the above problem, an uneven image density reading unit is arranged in an image forming apparatus, and an uneven image density distribution within the array of recording elements is periodically read to rewrite an unevenness correction data, thus providing an effective countermeasure. According to this technique, even if the uneven image density distribution of the head is changed, the unevenness correction data is rewritten accordingly, thereby always obtaining a uniform image free from unevenness. Such an image recording apparatus is proposed by the present applicant (U.S. Ser. No. 480,041 filed on Feb. 14, 1990; and U.S. Ser. No. 516,129 filed on Apr. 27, 1990).
A schematic arrangement obtained by applying the above proposal to an ink-jet recording apparatus is shown in FIG. 2.
Multi-nozzle ink heads 1C, 1M, 1Y, and 1Bk are of a cyan ink, a magenta ink, a yellow ink, and a black ink, respectively. Each ink-jet head has a density of 400 dpi, the number of nozzles as 4,736, and a width of about 300 mm and can selectively inject an ink (i.e., so-called on-demand scheme) for a recording material of a fixed A3 size (latitude: 297 mm) or less such as a recording material of a B4 or A4 size upon movement of the recording material. An image can be formed on the entire surface of the recording material upon a single movement of the recording material.
This ink-jet head can be realized by an ink-jet head for supplying electrical pulses to heating resistor elements formed utilizing, e.g., a semiconductor fabrication process, to heat an ink so as to form a bubble, thereby injecting or ejecting an ink droplet by its pressure. By utilizing four of these ink-jet heads, a high-speed full-color image recording apparatus having a speed of about 30 cpm (copies per minute) can be arranged. The full-color image recording apparatus includes a cassette 602 for storing recording materials 603 (not shown in FIG. 2). The recording material 603 (not shown in FIG. 2) is picked up by a pickup roller 604 and is electrostatically attracted to a conveyor belt 608 through first resist rollers 605, a guide plate 606, and second resist rollers 607, so that ink-jet recording is performed on a platen 609.
When normal ink-jet recording is to be performed, a test pattern reading system consisting of a lamp source 610 and an optical sensor 611 is not operated. The recording material on which a desired image is recorded is discharged onto a discharge tray 614 through a guide plate 612 and discharge rollers 613.
When ink-jet recording of a test pattern for correcting an uneven image density is to be performed, the test pattern reading system consisting of the lamp source 610 and the optical sensor 611 is operated. More specifically, the lamp source 610 is turned on, and the optical sensor 611 receives light reflected by the recorded test pattern and outputs an electrical signal proportional to a light reception quantity. Each of the lamp source 610 and the optical sensor 611 has a width equal to or larger than that of the ink-jet head 1. Alternatively, the lamp source 610 and the optical sensor 611 may be arranged such that scanning is performed in a direction perpendicular to the drawing surface (FIG. 2) along a guide rail (not shown) to read the recording characteristics of a plurality of recording elements (a plurality of nozzles in this case).
FIG. 3 shows an uneven image density reading unit 506 used in the above method. A recording medium 501 has an unevenness measurement test pattern. The reading unit 506 includes a light source 502 for illuminating a surface of the recording medium, a reading sensor 503 for reading light reflected by the surface of the recording medium, and lenses 504 and 505. The reading unit 506 having the above arrangement is scanned to read an unevenness distribution, thereby rewriting unevenness correction data.
FIG. 4 shows another uneven image density reading unit. The reading unit comprises a line sensor 520 comprising a CCD or the like having read pixels 521, and an unevenness correction test pattern 524 in which a recording element is formed by a width d in the y direction. The density of the test pattern is read by a recording head while the line sensor 520 is scanned in the x direction. Data read by the pixels 521 of the line sensor 520 correspond to density data formed by the respective recording elements of the recording head.
An algorithm of uneven image density correction proposed above will be briefly described with reference to FIGS. 5A to 5C and FIGS. 6A and 6B.
A test pattern (FIG. 5A) obtained by driving a plurality of recording elements (nozzles in this case) under the same condition, i.e., by the same drive image signal (the drive signal is defined as S.sub.0 in this case) is recorded. An optical density of the test pattern is not uniform due to variations caused by fabrication of the respective recording elements and variations caused by deteriorations over time, as shown in FIG. 5B. An uneven image density is caused, as shown in FIG. 5C. A drive signal S for the recording head in response to the read signal is corrected in units of recording elements in accordance with the read signal, thereby preventing an uneven image density.
More specifically, light is incident on the test pattern (FIG. 5A), and light reflected by the test pattern is received. The received light quantity is A/D-converted into digital data, thereby measuring a distribution Ei (where i is a recording area of the ith nozzle) (S.sub.0) of quantities of reflected light. (S.sub.0) represents that Ei is a function of S. If S=S.sub.0, then Ei is obtained. Light quantity to density conversion is performed to convert the distribution Ei(S.sub.0) of quantities of reflected light into a distribution ODi(S.sub.0) optical densities. An average density OD(S.sub.0) is calculated, and a reciprocal ratio OD(S.sub.0)/ODi(S.sub.0) for the average density OD(S.sub.0) of the optical density ODi(S.sub.0) of each portion is multiplied with the drive signal for driving the ith recording element, thereby correcting the uneven image density (to be described later), thereby obtaining a uniform image free from an uneven image density.
An example of the above operation will be briefly described below. Assume that a proportional relationship between the drive signal S and the optical density ODi(S) of the recording material is established, as shown in FIG. 6A. If no proportional relationship is established, the drive signal S is corrected so as to obtain a proportional relationship by using a look-up table or the like. Since all the recording elements are driven by the same drive signal S.sub.0, optical densities vary depending on the respective recording elements (i values), as shown in FIG. 6A. The second recording element, i.e., i=2 will be taken as an example (FIG. 6B). The optical density of a portion to be recorded by the second recording element is OD.sub.2 (S.sub.0)=a.sub.2 .times.S.sub.0 as opposed to the average density OD(S.sub.0)=a.times.S of the entire image, so that the portion to be recorded by the second recording element has a lower density than the average density. The drive signal for driving the second recording element is corrected to OD(S.sub.0)/OD.sub.2 (S.sub.0).times.S.sub.0. Since condition OD(S.sub.0)/OD.sub.2 (S.sub.0)=(a.times.S.sub.0)/(a.sub.2 .times.S.sub.0)=a/a.sub.2 is established, the drive signal for driving the second recording element is given as OD(S.sub.0)/OD.sub.2 (S.sub.0).times.S.sub.0 =a/a.sub.2 .times.S.sub.0. This correction value is proved to be a correct correction value (correction value for eliminating an uneven image density) as follows.
As is apparent from FIG. 6B, since .DELTA.ABC.alpha..DELTA.ADE is established, EQU BC:DE=AC:AE EQU BC:DE=a.sub.2 .times.S.sub.0 :a.times.S.sub.0 =a.sub.2 :a EQU and EQU AC:AE=a.sub.2 :a, AC=S.sub.0 EQU then EQU AE=a/a.sub.2 .times.S.sub.0
When the second recording element is driven with a/a.sub.2 .times.S.sub.0, the optical density is given as a.times.S.sub.0 which is equal to the average density from FIG. 6B. The unevenness of this portion can be apparently corrected. A correction value of the drive signal for driving the second recording element is thus confirmed to be OD(S.sub.0)/OD.sub.2 (S.sub.0) (this value is multiplied with the drive signal). This can also be applied to other recording elements. When a correction value OD/OD.sub.i is multiplied with a drive signal for driving the ith recording element, the uneven image density of the recorded image can be corrected and made uniform. Since any S.sub.0 value can be selected, the uneven image densities of all the drive signal values can be corrected and made uniform.
In summary, light is incident on the test pattern shown in FIG. 5A and a quantity of light reflected by the test pattern is measured. A correction quantity is calculated by performing the above calculations, and the recording head is driven by the corrected drive signals. The uneven image densities caused by variations in recording elements can be corrected, and a desired recorded image can be obtained.
In an image recording apparatus for performing image recording using the cyan, magenta, yellow, and black recording heads described above, recording characteristics of the recording heads are to be often detected and then an uneven image density is to be corrected. When light is incident on test patterns (of different colors) recorded by the respective recording heads, and conversion parameters for converting signals proportional to the quantities of reflected light into signals proportional to optical densities, are common to all the color components (i.e., cyan, magenta, yellow, and black), the quantities of reflected light cannot be accurately transformed into optical densities. For this reason, the uneven image density cannot be perfectly corrected.
When light is to be incident on the test patterns (of different colors) recorded by the respective recording heads, and analog data proportional to the distribution of the quantities of reflected light are to be converted into digital data, reference analog values for A/D conversion for cyan, magenta, yellow, and black recording heads are the same. For this reason, resolutions (representing a minimum density difference determination range) for the densities of the read data of the uneven image densities of the yellow, magenta, and cyan recording heads are decreased, and uneven image density correction precision is degraded, resulting in inconvenience.
In the above operations, it is generally difficult to actually obtain an image perfectly free from an uneven image density by a single calculation cycle. The calculations must be repeated until an image perfectly free from the uneven image density is obtained. When the operations are terminated after satisfactory unevenness density correction of an image with the resultant correction data is confirmed, a sufficient unevenness correction effect can be obtained.
Even if correction data is obtained by the above method, a sufficiently effective correction effect often cannot be obtained for an image if a large difference exists between the image and the test image used in unevenness correction.
For example, when correction data is rewritten by using a halftone image 50% duty this data is effective for the unevenness correction effect in 50% halftone. However, this data may not be effective for halftone of 0 to 15% or 75 to 85% because the gradation characteristics of the recording head are not necessarily linear, as those shown in FIG. 6A. The effect obtained by correction shown in FIG. 6B is not always valid throughout the entire range of the input signal.
If an image forming apparatus includes an image reading apparatus, converts an original image into an electrical signal, and performs image recording in accordance with the electrical signal, the original image reading apparatus can be conveniently used as a reading unit for the uneven image density of a test image without any modifications.
In an image forming apparatus having such an image reading apparatus, and particularly, an apparatus capable of reading or forming a color image, the reading apparatus outputs red (R), green (G), and blue (BL) signals. In this case, it is possible to read all color patterns.
Unevenness is a delicate phenomenon, and it is preferable to read each test image with high precision. In a normal copying mode, i.e., in a mode for reading an original image and copying the read image by a recording head, it is preferable not to sacrifice color reproducibility.
Another uneven image density reading head 506 used in the above method is shown in FIG. 7. This head comprises a lamp 502, a photo-diode 503, and lenses 504 and 505. The head can read an unevenness measurement pattern printed on recording sheet 501.
Light from the lamp 502 is collimated by the lens 504, and the collimated light is incident on an unevenness measurement pattern on the recording sheet 501. Light reflected by the recording sheet 501 is incident on the photo-diode 503 through the lens 505 and an aperture 507 having an opening of size d.sub.0. At this time, light incident on the photo-diode 503 falls within the range of d.sub.1 on the unevenness measurement pattern, so that an average value of the unevenness pattern within the range of d.sub.1 is detected. When unevenness correction data is rewritten on the basis of this detection result, a uniform image can always be obtained.
Still another uneven image density reading method will be described with reference to FIG. 8. A recording sheet and a CCD are denoted by reference numerals 520 and 521. An unevenness correction pattern printed by a multi-element head having a total width d of recording elements in the y direction. In this case, the number of recording elements in the multi-element head is 256, and the number of read pixels of the CCD is also 256. The density of a pattern printed with the multi-element head is read while the CCD is scanned in the direction of arrow 525.
Data read by the pixels of the CCD correspond to densities of the respective recording elements of the multi-element head, respectively.
When unevenness correction data are formed directly using the pixel data, an image contains a large number of noise components for the following reason.
Assume that dots D are formed in an uneven image density correction pattern in FIG. 9. The dot sizes are uneven in the x direction because diameters of dots recorded by one recording element of the multi-element head are changed. Density measurement results vary greatly depending on measurement timings. The dots are not equidistantly arranged in the y direction because dot recording position precision varies in the y direction. At this time, when a measurement is performed, a density corresponding to a reading element 521b is high, and a density corresponding to a reading element 521c is low.
For this reason, the read data of a predetermined area in the x and y directions are averaged, and the average value serves as data of the central reading pixel of this area. Unevenness correction is performed on the basis of the data obtained as described above, and a uniform image can always be obtained.
However, when an aperture size is fixed, as shown in FIG. 7, or when the size of the data averaging area is fixed, as shown in FIG. 8, the following drawback is present.
The aperture size and the size of the data averaging area are important factors in unevenness reading. When the size of the data averaging area is excessively large, a complicated unevenness pattern cannot be read. In this case, it is difficult to correct fine unevenness having a short period. To the contrary, when the data averaging area is too large, noise in a read pattern is extracted, and stable unevenness correction cannot be performed. Therefore satisfactory uniform image cannot be obtained by a single unevenness correction cycle. In order to obtain a satisfactory image, read and unevenness correction must be repeated a plurality of times.
For this reason, the aperture size and the size of the data averaging area must be set to optimal values. However, these optimal values change in accordance with print conditions. Assume that a large image is output and used for a poster or the like. In this case, since the poster is not observed from a position close to it, fine unevenness is not so conspicuous, so that correction of an uneven image density having a long period is the primary concern. In this sense, the aperture size and the size of the data averaging area are preferably large. On the other hand, in the case of a small image which is observed close at hand, elimination of fine unevenness becomes the primary concern. In this case, it is preferable to reduce the aperture size and the size of the data averaging area.
In this manner, an optimal aperture size and an optimal size of a data averaging area vary depending on output images.
Optimal values also change depending on time required for forming unevenness correction data.
In the market, servicemen often replace an old head with a new one. At this time, he forms unevenness correction data. In this case, it is better to the time to perform high-precision unevenness correction, and the aperture size and the size of the data averaging area are preferably set to be small.
When an ordinary user performs unevenness correction during use, unevenness correction of a given level or more can be performed within a short period of time by setting the aperture size and the size of the data averaging area to be large.
These optimal values are also changed depending on the number of head-use times.
When a new head is used immediately after its replacement, print position precision of each dot is relatively good. Read noise caused by variations in print position precision, shown in FIG. 9, is small. The aperture size and the size of the data averaging area can be small. In an old head, however, since print position precision of each dot has degraded, the aperture size and the size of the averaging area are preferably set to be large.
Optimal values of the aperture size and the size of the data averaging area vary depending on various conditions. Fixed values for the size of the aperture and data averaging area cannot cope with various conditions.
A read speed and a focus state of a read optical system are used as factors having the same effects as the aperture size and the size of the data averaging area.
When the read speed is high, fine unevenness tends not to be read. However, when the read speed is low, high-precision reading can be performed, although noise in a pattern tends to be picked up.
Fine unevenness tends not to be read when a defocus state is set. However, when an in-focus state is set, noise in the pattern tends to be picked up.
When predetermined read conditions are set, it is difficult to perform optimal reading.
Types of unevenness correction patterns, and a method of calculating unevenness correction data in addition to the various read conditions are often preferably changed depending whether fine unevenness correction requiring a long period of time, or coarse unevenness correction requiring a short period of time is appropriate. For example, precision can be improved by correcting a plurality of patterns instead of correcting one type of pattern because a calculation method suitable for a specific purpose must be used. When there is only one method of calculating unevenness correction patterns, optimal unevenness correction data suitable for various conditions cannot be formed.